The present invention relates to a helium cooling apparatus in which gas helium in a liquid-helium container is cooled to be recondensed, and more particularly to a helium cooling apparatus in which a condensation-heat exchanger in the liquid-helium container has an improved heat transfer coefficient.
Conventionally, a liquid-helium container for cooling a superconducting coil and the like is disposed adiabatically in a cryostat. A helium cooling apparatus is used to cool and recondense gas helium in the liquid-helium container. To attain this, the cooling apparatus comprises a refrigerator for cooling a refrigerant, and a condensation-heat exchanger for evaporating the refrigerant to cool the gas helium. In general, helium cooling apparatuses can be classified into two types. In one type, the refrigerator is incorporated in the cryostat, and the condensation-heat exchanger is located in the liquid-helium container. In the other type, an exclusive-use cylindrical member extends from an exclusive-use port in the liquid-helium container to the outside of the cryostat. The heat exchanger is inserted into the helium container through the port and the cylindrical member for exclusive use. The refrigerator is disposed inside the cylindrical member or outside the cryostat.
In maintaining the refrigerator, in the case of the first type, the refrigerator must be disassembled, repaired, and reassembled after the temperature of the helium in the liquid-helium container is raised. In this type, therefore, the refrigerator cannot be maintained with ease.
In the case of the second type, on the other hand, the helium cooling apparatus can be mounted or demounted easily, without causing the liquid helium in the container to be discharged. In the second type, therefore, the refrigerator can be maintained without increasing the temperature of the helium in the helium container. Thus, as regards the maintenance of the refrigerator, the helium cooling apparatus of the second type has an advantage over the first type.
The performance of the helium cooling apparatus depends on that of the refrigerator and the heat transfer coefficient of the condensation-heat exchanger. In order to improve the performance of the cooling apparatus, therefore, the heat transfer coefficient of the exchanger must be improved. Thus, the heat-transfer area of the heat exchanger is expected to be increased.
In the helium cooling apparatus of the second type, however, the diameters of the port and the cylindrical member for exclusive use depend on the size of the condensation-heat exchanger. If the heat-transfer area of the heat exchanger becomes greater, therefore, the diameter of the exchanger, and hence, those of the port and the cylindrical member, are increased in proportion. Thus, the amount of heat introduced into the liquid-helium container, through the port and the cylindrical member, increases. The introduced heat lowers the thermal efficiency of the whole cooling apparatus.
Since the diameter of the prior art condensation-heat exchanger is considerably large, moreover, the helium cooling apparatus of the second type cannot be applied to a liquid-helium container without an exclusive-use port.